Components with environmental barrier coatings having improved surface roughness

ABSTRACT

Components having an environmental barrier coating and a sintered layer overlying the environmental barrier coating, the sintered layer defining an outer surface having a lower surface roughness than the environmental barrier coating. The sintered layer is formed from a slurry applied to and then sintered on the environmental barrier coating. The sintered layer comprises a primary material, at least one sintering aid dissolved in the primary material, and optionally a secondary material. The sintering aid contains at least one doping composition. The primary material is a rare earth disilicate or a rare earth monosilicate and is doped with the doping composition so as to be either a doped rare earth disilicate or a doped rare earth monosilicate. The optional secondary material is a reaction product of the primary material and any of the sintering aid not dissolved in the primary material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 12/642,317filed Dec. 18, 2009, which claims priority to U.S. ProvisionalApplication Ser. No. 61/230,262, filed Jul. 31, 2009. The contents ofthese prior patent applications are incorporated herein by reference intheir entirety.

BACKGROUND OF THE INVENTION

Embodiments described herein generally relate to methods of improvingsurface roughness of an environmental barrier coating and componentscomprising environmental barrier coatings having improved surfaceroughness. More particularly, embodiments described herein generallyrelate to improving surface roughness of environmental barrier coatingsusing water-based slurries comprising at least one sintering aid.

Higher operating temperatures for gas turbine engines are continuouslybeing sought in order to improve their efficiency. However, as operatingtemperatures increase, the high temperature durability of the componentsof the engine must correspondingly increase. Significant advances inhigh temperature capabilities have been achieved through the formulationof iron, nickel, and cobalt-based superalloys. While superalloys havefound wide use for components used throughout gas turbine engines, andespecially in the higher temperature sections, alternativelighter-weight component materials have been proposed.

Ceramic matrix composites (CMCs) are a class of materials that consistof a reinforcing material surrounded by a ceramic matrix phase. Suchmaterials, along with certain monolithic ceramics (i.e. ceramicmaterials without a reinforcing material), are currently being used forhigher temperature applications. These ceramic materials are lightweightcompared to superalloys yet can still provide strength and durability tothe component made therefrom. Therefore, such materials are currentlybeing considered for many gas turbine components used in highertemperature sections of gas turbine engines, such as airfoils (e.g.turbines, and vanes), combustors, shrouds and other like components thatwould benefit from the lighter-weight and higher temperature capabilitythese materials can offer.

CMC and monolithic ceramic components can be coated with environmentalbarrier coatings (EBCs) to protect them from the harsh environment ofhigh temperature engine sections. EBCs can provide a dense, hermeticseal against the corrosive gases in the hot combustion environment,which can rapidly oxidize silicon-containing CMCs and monolithicceramics. Additionally, silicon oxide is not stable in high temperaturesteam, but rather, can be converted to volatile (gaseous) siliconhydroxide species. Thus, EBCs can help prevent dimensional changes inthe ceramic component due to such oxidation and volatilizationprocesses. Unfortunately, there can be some undesirable issuesassociated with standard, industrial coating processes such as plasmaspray and vapor deposition (i.e. chemical vapor deposition, CVD, andelectron beam physical vapor deposition, EBPVD) currently used to applyEBCs. As an example, plasma spray processes can result in an EBC havinga surface roughness of greater than 200 micro inch (about 5 micrometers)Ra, which can be undesirable when factoring in aerodynamic designconsiderations in advanced turbine engines.

Accordingly, there remains a need for environmental barrier coatings toprotect CMCs from the high temperature steam environments present in gasturbine engines, including methods for improving the surface roughnessof plasma sprayed EBCs.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments herein generally relate to components comprising a plasmasprayed environmental barrier coating having a surface characterized bya surface roughness, and a sintered layer overlying the environmentalbarrier coating and defining an outer surface having a lower surfaceroughness than the surface of the environmental barrier coating.

According to one aspect of the invention, the sintered layer is formedfrom a slurry applied to and then sintered on the plasma sprayedenvironmental barrier coating. The sintered layer comprises a primarymaterial, at least one sintering aid dissolved in the primary material,and optionally a secondary material. The at least one sintering aidcontains at least one doping composition chosen from the groupconsisting of iron, aluminum, titanium, gallium, nickel, boron, alkalimetals, alkaline-earth metals, and Lnb rare earth metals, and isselected from the group consisting of iron oxide, gallium oxide,aluminum oxide, nickel oxide, titanium oxide, boron oxide, alkalineearth oxides, carbonyl iron, iron metal, aluminum metal, boron, nickelmetal, iron hydroxide, gallium hydroxide, aluminum hydroxide, nickelhydroxide, titanium hydroxide, alkaline earth hydroxides, ironcarbonate, gallium carbonate, aluminum carbonate, nickel carbonate,boron carbonate, alkaline earth carbonates, iron oxalate, galliumoxalate, aluminum oxalate, nickel oxalate, titanium oxalate, watersoluble iron salts, water soluble gallium salts, water soluble aluminumsalts, water soluble nickel salts, water titanium salts, water solubleboron salts, water soluble alkaline earth salts, and a combinationcomprising an Lnb rare earth metal and SiO₂. The primary material is arare earth disilicate or a rare earth monosilicate, and is doped withthe at least one doping composition of the at least one sintering aid soas to be either a doped rare earth disilicate containing the at leastone doping composition or a doped rare earth monosilicate containing theat least one doping composition. The secondary material optionally inthe sintered layer is a reaction product of the primary material and anyof the at least one sintering aid not dissolved in the primary material.

These and other features, aspects and advantages will become evident tothose skilled in the art from the following disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the invention, it is believed that theembodiments set forth herein will be better understood from thefollowing description in conjunction with the accompanying figures, inwhich like reference numerals identify like elements.

FIG. 1 is a schematic cross sectional view of one embodiment of acomponent having and environmental barrier coating in accordance withthe description herein.

FIG. 2 is a SEM cross-section of an EBC coating on a SiC—SiC CMC inaccordance with Example 1 herein.

FIG. 3 is a close up view of FIG. 2 in accordance with Example 1 herein.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments described herein generally relate to water basedenvironmental barrier coatings for high temperature ceramic components.More particularly, embodiments herein generally describe water basedenvironmental barrier coatings comprising sintering aids for use on hightemperature ceramic components.

More specifically, the EBCs described herein comprise sintering aids,which can lower the sintering temperature, thereby promoting theformation of dense EBC layers that can act as a hermetic seal to protectthe underlying component from corrosion from the gases generated duringhigh temperature combustion without damaging the component throughexposure to high sintering temperatures, as explained herein below.

The EBCs described herein may be suitable for use in conjunction withCMCs or monolithic ceramics. As used herein, “CMCs” refers tosilicon-containing matrix and reinforcing materials. Some examples ofCMCs acceptable for use herein can include, but should not be limitedto, materials having a matrix and reinforcing fibers comprising siliconcarbide, silicon nitride, and mixtures thereof. As used herein,“monolithic ceramics” refers to materials comprising silicon carbide,silicon nitride, and mixtures thereof. Herein, CMCs and monolithicceramics are collectively referred to as “ceramics.”

As used herein, the term “barrier coating(s)” can refer to environmentalbarrier coatings (EBCs). The barrier coatings herein may be suitable foruse on “ceramic component,” or simply “component” 10 found in hightemperature environments (e.g. operating temperatures of above 2100° F.(1149° C.)), such as those present in gas turbine engines. Examples ofsuch ceramic components can include, for example, combustor components,turbine blades, shrouds, nozzles, heat shields, and vanes.

More specifically, EBC 12 may comprise a coating system includingvarious combinations of the following: a bond coat layer 14, an optionalsilica layer 15, at least one transition layer 16, an optional compliantlayer 18, an optional intermediate layer 22, and an optional outer layer20, as shown generally in FIG. 1 and as set forth herein below.

Bond coat layer 14 may comprise silicon metal, silicide, or acombination thereof, and may generally have a thickness of from about0.1 mils to about 6 mils (about 2.5 to about 150 micrometers). Due tothe application method as described herein below, there may be somelocal regions where the silicon bond coat is missing, which can beacceptable. For example, in one embodiment, bond coat layer can coverabout 100% of the surface of the component, and in another embodiment,about 90% or more of the surface area of the component. As used herein“silicide” may include rare earth (Ln) silicides, chromium silicide(e.g. CrSi₃), niobium silicide (e.g. NbSi₂, NbSi₃), molybdenum silicide(e.g. MoSi₂, Mo₅Si₃, MoSi₃), tantalum silicide (e.g. TaSi₂, TaSi₃),titanium silicide (e.g. TiSi₂, TiSi₃), tungsten silicide (e.g. WSi₂,W₅Si₃), zirconium silicide (e.g. ZrSi₂), and hafnium silicide (e.g.HfSi₂).

As used herein, “rare earth” represented “(Ln)” refers to the rare earthelements of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce),praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), andmixtures thereof.

Silica layer 15 can be amorphous or crystalline, and have an initialthickness of from about 0.0 mils to about 0.2 mils (about 0.0 to about 5micrometers). However, the thickness of silica layer 15 can increaseover time. Specifically, the silicon in bond coat layer 14 can oxidizeslowly during the service life of the EBC to gradually increase thethickness of silica layer 15. This oxidation of bond coat 14 can protectthe underlying ceramic component from oxidation since the bond coat isoxidized rather than the ceramic component. Silica layer 15 can, in someembodiments, also be doped with a doping composition, as defined hereinbelow, due to diffusion of the sintering aid into the silica layer.

Transition layer 16 may comprise a rare earth disilicate, a doped rareearth disilicate, or a doped rare earth disilicate containing secondarymaterials, as defined below. More specifically, transition layer 16 mayinclude from about 85% to about 100% by volume of the transition layerof a primary transition material and up to about 15% by volume of thetransition layer of a secondary material, and in one embodiment fromabout 85% to about 99% by volume of the transition layer of the primarytransition material and from about 1% to about 15% by volume of thetransition layer of the secondary material. In another embodiment,transition layer 16 may comprise 100% primary transition materialwherein the primary transition material can be doped, as describedbelow.

As used herein, “primary transition material” refers to a rare earthdisilicate (Ln₂Si₂O₇), or a doped rare earth disilicate. As used herein,“doped rare earth disilicate” refers to Ln₂Si₂O₇ doped with a “dopingcomposition” selected from the group consisting of iron (Fe), aluminum(Al), titanium (Ti), gallium (Ga), nickel (Ni), boron (B), an alkali, analkali-earth, and Lnb rare earths, as defined herein below. As usedherein throughout, “secondary material” refers to a material comprisinga doping composition (as defined previously), and specifically, can beselected from the group consisting of Fe₂O₃, iron silicates, rare earthiron oxides, Al₂O₃, mullite, rare earth aluminates, rare earthaluminosilicates, TiO₂, rare earth titanates, Ga₂O₃, rare earthgallates, NiO, nickel silicates, rare earth nickel oxides, Lnb metals,Lnb₂O₃, Lnb₂Si₂O₇, Lnb₂SiO₅, borosilicate glass, alkaline earthsilicates (for example, barium-strontium-aluminosilicates (BSAS)),alkaline earth rare earth oxides, alkaline earth rare earth silicates,and mixtures thereof. Any doping composition present in the primarymaterial should correspond to the doping composition contained in anysecondary material present (e.g. Fe-doped Ln₂Si₂O₇ with Fe₂O₃ secondarymaterial; Ti-doped Ln₂Si₂O₇ with TiO₂ secondary material; or Ni-dopedLn₂Si₂O₇ with rare earth nickel oxide secondary material, for example).

Each transition layer 16 may have a thickness of from about 0.1 mils toabout 40 mils (about 2.5 micrometers to about 1 millimeter), and may bemade and applied to the underlying layer as set forth below. In oneembodiment, there may be more than one transition layer present. In suchinstances, each transition layer may comprise the same or differentcombination of primary transition materials and secondary materials.Transition layer 16 may have a porosity level of from 0% to about 15% byvolume of the transition layer, and in another embodiment, from about0.01% to about 15% by volume of the transition layer.

Similarly, outer layer 20 may comprise a rare earth monosilicate, adoped rare earth monosilicate, or a doped rare earth monosilicatecontaining secondary material. More specifically, outer layer 20 caninclude from about 85% to about 100% by volume of the outer layer of aprimary outer material and up to about 15% by volume of the outer layerof the previously defined secondary material, and in one embodiment fromabout 85% to about 99% by volume of the outer layer of a primary outermaterial and from about 1% to about 15% by volume of the outer layer ofthe secondary material. In another embodiment, outer layer 20 maycomprise 100% primary outer material wherein the primary outer materialcan be doped as described below.

As used herein, “primary outer material” refers to a rare earthmonosilicate, or a doped rare earth monosilicate. As used herein, “dopedrare earth monosilicate” refers to Ln₂SiO₅ doped with a dopingcomposition, as defined previously. Outer layer 20 may have a thicknessof from about 0.1 mils to about 3 mils (about 2.5 to about 75micrometers), and may be made and applied to the underlying layer as setforth below. In one embodiment, outer layer 20 may have a porosity levelof from 0% to about 30% by volume of the outer layer, and in anotherembodiment, from about 0.01% to about 30% by volume of the outer layer,and in another embodiment, from about 0.01% to about 15% by volume ofthe outer layer. In some embodiments, outer layer 20 can comprise crackstherein at a density of up to about 10 cracks/mm that can form duringoperation due to thermal expansion anisotropy.

In reference to the embodiments herein, “Lnb rare earth (metal)”, orsimply “Lnb” refers to a sub-set of rare-earth metals having a meltingpoint below at least about 1450° C. including lanthanum, cerium,praseodymium, neodymium, promethium, samarium, europium, gadolinium,terbium, dysprosium, and ytterbium. In one embodiment, the sub-set caninclude only those rare earth elements having a melting point belowabout 1350° C. including lanthanum, cerium, praseodymium, neodymium,promethium, samarium, europium, gadolinium, and ytterbium. The Lnb rareearth metal can be utilized with SiC—SiC CMCs having an operation limitof about 1357° C.

As used herein throughout, “alkaline earth” can refer to magnesium (Mg),calcium (Ca), strontium (Sr), and barium (Ba). As used herein, “alkali”refers to lithium (Li), potassium (K), and sodium (Na). “Iron silicates”can include compounds such as Fe₂SiO₄, and glasses of rare earth ironsilicates. “Rare earth iron oxides” can include compounds such asgarnets (Ln₃Fe₅O_(i2)), monoclinic ferrites (Ln₄Fe₂O₉), and perovskites(LnFeO₃). “Rare-earth aluminates” can include compounds such as garnets(Ln₃Al₅O₁₂), monoclinic aluminates (Ln₄Al₂O₉), and perovskites (LnAlO₃).“Rare earth aluminosilicates” can include glassy materials comprised ofabout 35-50 wt % Ln₂O₃, about 15-25 wt % Al₂O₃, and about 25-50 wt %SiO₂. “Rare-earth titanates” can include compounds such as Ln₂Ti₂O₇(pyrochlore) and Ln₂TiO₅. “Rare-earth gallates” can include compoundssuch as garnets (Ln₃Ga₅O_(i2)), monoclinic gallates (Ln₄Ga₂O₉),perovskites (LnGaO₃), and Ln₃GaO₆. “Nickel silicates” can includecompounds such as Ni₂SiO₄. “Borosilicate glass” can refer to anyamorphous material containing up to about 15% by weight boron oxide(B₂O₃), up to about 10% alkali oxide selected from the group consistingof sodium (Na₂O), potassium (K₂O), lithium (Li₂O), or any combinationsof thereof, up to about 10% alumina (Al₂O₃), and a balance of silica(SiO₂). “Alkaline earth silicates” can include compounds such asMg₂SiO₄, MgSiO₃, Ca₂SiO₄, Ca₃SiO₅, Ca₃Si₂O₇, CaSiO₃, Ba₂SiO₄, BaSiO₃,Ba₂Si₃O₈, BaSi₂O₅, Sr₂SiO₄, and SrSiO₃. “Alkali earth rare earth oxides”can include compounds such as BaLn₂O₄, Mg₃Ln₂O₆, SrLn₂O₄, and Sr₂Ln₂O₅.“Alkaline earth rare earth silicates” can include oxyapatite materials(i.e. Ae₂Ln₈Si₆O₂₆).

If present, compliant layer 18 may include from about 85% to about 100%by volume of the compliant layer of a primary compliant material and upto about 15% by volume of the compliant layer of a secondary compliantmaterial, and in one embodiment from about 85% to about 99% by volume ofthe compliant layer of a primary compliant material and from about 1% toabout 15% by volume of the compliant layer of the secondary compliantmaterial. In another embodiment, compliant layer 18 may comprise 100% byvolume of the compliant layer of a primary compliant material whereinthe primary compliant material may be doped with a rare earth element.

As used herein, “primary compliant material” refers to BSAS, or a rareearth doped BSAS, while “secondary compliant material” refers to Ln₂O₃,Ln₂Si₂O₇, Ln₂SiO₅, Ln₃Al₅O₁₂, Al₂O₃, mullite, and combinations thereof.Compliant layer 20 may have a thickness of from about 0.1 mils to about40 mils (about 2.5 micrometers to about 1 millimeter), and may be madeand applied as set forth below. In one embodiment, compliant layer 18may have a porosity level of from 0% to about 30% by volume of thecompliant layer, and in another embodiment, from about 0.01% to about30% by volume of the compliant layer, and in another embodiment, fromabout 0.01% to about 15% by volume of the compliant layer.

Intermediate layer 22, if present, can comprise the previously definedprimary outer materials of rare earth monosilicate or doped rare earthmonosilicate. Similar to the silica layer, intermediate layer 22 canform during the service life of the EBC. More specifically, hightemperature steam penetrates the outer layer 20, and as the steam reactswith the primary transition material of the transition layer tovolatilize SiO2, intermediate layer 22 can form.

By way of example, and not limitation, the EBC systems described hereinmay include in one embodiment, component 10, bond coat layer 14, andtransition layer 16; in another embodiment, component 10, bond coatlayer 14, transition layer 16, and outer layer 20; in anotherembodiment, component 10, bond coat layer 14, transition layer 16,compliant layer 18, and outer layer 20; in another embodiment, component10, bond coat layer 14, transition layer 16, compliant layer 18,transition layer 16, and outer layer 20; in another embodiment,component 10, bond coat layer 14, silica layer 15, and transition layer16; in another embodiment, component 10, bond coat layer 14, silicalayer 15, transition layer 16, and outer layer 20; in anotherembodiment, component 10, bond coat layer 14, silica layer 15,transition layer 16, compliant layer 18, and outer layer 20; in anotherembodiment, component 10, bond coat layer 14, silica layer 15,transition layer 16, compliant layer 18, transition layer 16, and outerlayer 20; in another embodiment, component 10, bond coat layer 14,transition layer 16, intermediate layer 22, and outer layer 20; inanother embodiment, component 10, bond coat layer 14, silica layer 15,transition layer 16, intermediate layer 22, and outer layer 20; inanother embodiment, component 10, bond coat layer 14, silica layer 15,transition layer 16, intermediate layer 22 (which can form duringoperation), and outer layer 20; and in another embodiment, component 10,bond coat layer 14, silica layer 15, transition layer 16, compliantlayer 18, transition layer 16, intermediate layer 22 (which can formduring operation), and outer layer 20. Such embodiments can be suitablefor use in environments having a temperature up to about 1704° C. (3100°F.).

Alternately, the EBC system may comprise component 10, bond coat layer14, transition layer 16, and compliant layer 18; and in anotherembodiment, component 10, bond coat layer 14, silica layer 15,transition layer 16, and compliant layer 18. Such embodiments can besuitable for use in environments having a temperature of up to about1538° C. (2800° F.).

Those skilled in the art will understand that embodiments in addition tothose set forth previously are also acceptable, and that not all of thelayers need to be present initially, but rather, may form during engineoperation.

The EBC can be made and applied in accordance with the descriptionbelow.

Bond coat layer 14 may be applied by plasma spray processes, chemicalvapor deposition processes, electron beam physical vapor depositionprocesses, dipping in molten silicon, sputtering processes, and otherconventional application processes known to those skilled in the art.

As previously described, silica layer 15 can form during the servicelife of the EBC. Specifically, oxygen in the surrounding atmosphere candiffuse through any of the outer layer, compliant, and transitionlayer(s) present in the EBC and react with the silicon of bond coatlayer 14 to form silica layer 15. Alternately, silica layer 15 may beintentionally deposited by chemical vapor deposition, plasma spray,slurry deposition, or other conventional method.

Similar to silica layer 15, intermediate layer 22 can also form duringthe service life of the EBC when high temperature steam reacts withtransition layer 16, as previously described.

The manufacturing and application processes for transition layer 16,compliant layer 18 and outer layer 20 can consist of a slurry depositioncycle including sintering aids to lower the temperature needed todensify the layers. The slurry deposition cycle can generally includeslurry formation, slurry application, drying, and sintering, withoptional masking, leveling, sintering aid infiltration, mask removal,and binder burnout steps, as set forth below. Those skilled in the artwill understand that slurries of varying compositions can be used tomake EBC layers of varying composition and that multiple slurrydeposition cycles can be used to build up the total thickness of aparticular layer. Each layer can have the thickness set forth previouslywith the average thickness per slurry deposition cycle dependingprimarily on the slurry solids loading, sintering aid concentration, andnumber of dip, spray, or paint passes.

The slurries described in the embodiments herein can comprise variousslurry components, but generally include water, ceramic particles,sintering aid, and organic processing aids. Particularly, the slurry maycomprise from about 1 wt % to about 99.9 wt % water; from about 0 wt %to about 33 wt % of a dispersant; from about 0 wt % to about 7 wt % of aplasticizer; from about 0 wt % to about 1 wt % surfactant; from about 0wt % to about 25 wt % slurry sintering aid if there is one sinteringaid, or alternately, from about 0 wt % to about 79.9 wt % slurrysintering aid if there are two sintering aids present; and in anotherembodiment, from about 0.01 wt % to about 25 wt % slurry sintering aidif there is one sintering aid, or alternately, from about 0.01 wt % toabout 79.9 wt % slurry sintering aid if there are two sintering aidspresent; from about 0.1 wt % to about 72 wt % of primary material; fromabout 0 wt % to about 1 wt % of a thickener; from about 0 wt % to about20 wt % of a latex binder; and from about 0 wt % to about 11 wt % of asecondary additive for controlled dispersion.

More specifically, “dispersant” refers to polyacrylic acid, polyacrylicacid-polyethylene oxide copolymers, polyvinyl phosphoric acid,polymethacrylic acid, polyethylenimine, ammonium polyacrylate, ammoniumpolymethacrylate, sulfonated naphthalene formaldehyde condensate,polyvinyl sulfonic acid, and combinations thereof.

“Plasticizer” refers to ethylene glycol, diethylene glycol, triethyleneglycol, tetraethylene glycol glycerol, glycerin, polyethylene glycol,and combinations thereof.

“Surfactant” refers to compositions selected from the group consistingof fluorocarbons, dimethylsilicones, and ethoxylated acetylenic diolchemistries (e.g. commercial surfactants in the Surfynol® series such asSurfynol® 420 and 502 (Air Products and Chemicals, Inc.)), andcombinations thereof.

As used herein, “slurry sintering aid” can refer to sintering aidcompositions suitable for inclusion in the slurry. In some embodiments,there can be from about 0 wt % to about 25 wt %, and in some embodimentsfrom about 0.01 wt % to about 25 wt %, of a slurry sintering aidselected from iron oxide, gallium oxide, aluminum oxide, nickel oxide,titanium oxide, boron oxide, and alkaline earth oxides; carbonyl iron;iron metal, aluminum metal, boron, nickel metal, hydroxides includingiron hydroxide, gallium hydroxide, aluminum hydroxide, nickel hydroxide,titanium hydroxide, alkaline earth hydroxides; carbonates including ironcarbonate, gallium carbonate, aluminum carbonate, nickel carbonate,boron carbonate, and alkaline earth carbonates; oxalates including ironoxalate, gallium oxalate, aluminum oxalate, nickel oxalate, titaniumoxalate; and “water soluble salts” including water soluble iron salts,water soluble gallium salts, water soluble aluminum salts, water solublenickel salts, water titanium salts, water soluble boron salts, and watersoluble alkaline earth salts. In the case of the compliant layer slurry,the “slurry sintering aid” may include rare earth nitrate, rare earthacetate, rare earth chloride, rare earth oxide, ammonium phosphate,phosphoric acid, polyvinyl phosphoric acid, and combination thereof.

In an alternate embodiment, the slurry can comprise from about 0 wt % toabout 59.3 wt %, and in one embodiment from about 0.01 wt % to about59.3 wt %, of an Lnb rare earth metal slurry sintering aid as definedpreviously herein, and from about 0 wt % to about 20.6 wt %, and in oneembodiment from about 0.01 wt % to about 20.6 wt. %, of a SiO₂ slurrysintering aid. In this embodiment, the Lnb and SiO₂ content can be heldsuch that the mole ratio of Lnb to SiO₂ is about 1 to 1 for slurriescontaining rare earth disilicate primary transition material, and about2 to 1 for slurries containing rare earth monosilicate primary outermaterial.

As used herein, “water-soluble iron salts” can include iron nitrate andiron acetate; “water-soluble gallium salts” can include gallium nitrateand gallium acetate; “water-soluble aluminum salts” can include aluminumnitrate and aluminum acetate; “water-soluble nickel salts” can includenickel nitrate and nickel acetate; “water-soluble titanium salts” caninclude titanium chloride; “water-soluble boron salts” can include boricacid and ammonium borate; and “water-soluble alkaline earth salts” caninclude Mg(NO₃)₂, Ca(NO₃)₂, Sr(NO₃)₂, Ba(NO₃)₂, MgC₂H₃O₂, CaC₂H₃O₂,SrC₂H₃O₂, and BaC₂H₃O₂.

As defined previously, the “primary material” can be selected fromLn₂Si₂O₇, Ln₂SiO₅, or BSAS depending on which layer is being made.

“Secondary additives for controlled dispersion” include citric acid,glycine, dextrose, sucrose, mannose, tartaric acid, oxalic acid, andcombinations thereof.

“Thickener” refers to xanthan gum, polyethylene oxide, guar gum,polyacrylic acid, polyvinylpyrolidone, methylcellulose, and othercellulose derivatives, and combinations thereof.

“Latex binder” refers to polystyrene, polyvinyl alcohol, polyvinylbutyrol, styrene-butadiene copolymer, polyacrylic acid, polyacrylates,acrylic polymers, polymethyl methacrylate/polybutyl acrylate, polyvinylacetate, polyvinyl malate, and natural latex rubber. Some examples oflatex binders can include Rhoplex™ HA-8, Rhoplex™ HA-12, Pavecryl™ 2500(Rohm and Haas).

Also, as used herein, “organic processing aids” refers to dispersants,plasticizers, secondary additives for controlled dispersion, thickeners,and latex binders present in the slurry. These organic processing aidsare comprised primarily of carbon and other elements that volatilizeduring processing such that they are not present in the post-sinteredcoating.

The slurry can be formed by combining the previously described slurrycomponents with mixing media in a container. The mixture can be mixedusing conventional techniques known to those skilled in the art such asshaking with up to about a 1 inch (about 25.4 mm) diameter alumina orzirconia mixing media, ball milling using about a 0.25 inch to about a 1inch (about 0.64 cm to about 2.54 cm) diameter alumina or zirconiamixing media, attritor milling using about a 1 mm to about a 5 mmdiameter zirconia-based mixing media, planetary ball milling using fromabout a 1 mm to about a 5 mm diameter zirconia-based media, ormechanical mixing or stirring with simultaneous application ofultrasonic energy. The mixing media or ultrasonic energy can break apartany agglomerated ceramic particles in the slurry. Any mixing mediapresent may then be removed by straining, for example.

If not added previously, any of the remaining slurry components may beadded, in turn, along with mixing until the component dissolves, whichis generally after about 5 to about 60 minutes.

Once all slurry components have been mixed, the slurry can be filteredthrough screens of varying mesh sizes to remove any impurities that maybe present, such as after the initial mixing of the slurry or after useof the slurry to deposit coating layers. A 325 mesh screen, for example,can be used to filter out impurities having an average size of about 44microns or greater.

After mixing and optional filtering, the slurry can be agitatedindefinitely by slow rolling, slow mechanical mixing, or other likemethods to avoid trapping air bubbles in the slurry. In one embodiment,the slurry may be refreshed by adding additional water to account forthat which has evaporated during processing. Alternately, once mixed,the slurry can be set aside until needed for application. Those skilledin the art will understand that the previous embodiment sets forth onemethod for making the slurry compositions described herein, and thatother methods are also acceptable, as set forth in the Examples below.

Optionally, masking can be applied to the ceramic component before theslurry is applied to prevent coating specific areas of the component.Masking may be carried out using conventional techniques known to thoseskilled in the art including, but not limited to, tapes, tooling, andpaint-on adhesives.

Once all desired masking of the ceramic component is complete, theslurry can be applied to the component to produce a coated component.The slurry can be applied to the component (or on top of a previouslyapplied layer) using any conventional slurry deposition method known tothose skilled in the art, including but not limited to, dipping thecomponent into a slurry bath, or painting, rolling, stamping, spraying,or pouring the slurry onto the component. In one embodiment, slurryapplication can be carried out in a humid environment to help preventwater evaporation that could change the slurry rheology, for example,during coating deposition onto a large batch of parts. In oneembodiment, “humid environment” refers to greater than 50% relativehumidity, in another embodiment greater than 70% relative humidity, andin yet another embodiment greater than 95% relative humidity, all at ornear room temperature (about 20° C. to about 30° C.). Slurry applicationcan be carried out manually or it may be automated.

Once the slurry has been applied to the component, and while the slurryis still wet, it may be leveled to remove excess slurry material.Leveling may be carried out using conventional techniques such as, butnot limited to, spinning, rotating, slinging the component, drippingwith or without applied vibration, or using a doctor blade, to removeexcess slurry material. Similar to the slurry application, leveling canbe conducted manually or it may be automated, and it can be carried outin a humid environment because if the slurry dries too quickly it canlead to defects in the coating during leveling.

Next, the coated component can be dried to produce a dried component.Drying may be carried out in ambient or controlled temperature andhumidity conditions. In one embodiment, controlled temperature andhumidity can be utilized to help maintain the integrity of the appliedslurry coating. More particularly, in one embodiment, drying may becarried out at temperatures from about 5° C. to about 100° C., and inanother embodiment, from about 20° C. to about 30° C., and at a humidityrange of from about 10% relative humidity to about 95% relativehumidity, in one embodiment from about 50% relative humidity to about90% relative humidity, and in yet another embodiment from about 70%relative humidity to about 80% relative humidity.

After drying, any masking present may then be removed by peeling offtapes and adhesives, pyrolysis of tapes and adhesives, or removingmulti-use tooling. Any rough edges remaining after masking removal maybe scraped or cut away using a sharp or abrasive tool.

Next, burnout of the organic processing aids may be carried out byplacing the dried component in an elevated temperature environment sothat any bound water can be evaporated and the organic processing aidscan be pyrolyzed. In one embodiment, burnout of the organic processingaids may be accomplished by heating the dried component at a rate offrom about 1° C./min to about 15° C./min to a temperature of from about400° C. to about 1000° C. and holding the component at this temperaturefor from about 0 to about 10 hours. In another embodiment, the coatedcomponent may be heated at a rate of from about 2° C./min to about 6°C./min to a temperature of from about 600° C. to about 800° C. andholding the component at this temperature for from about 0 to about 10hours. In another embodiment, the hold time can be eliminated by slowlyramping up to the target temperature without holding, followed byramping up or down to another temperature at a different rate. Inanother embodiment, binder burnout can occur rapidly by placing thecoated component into a furnace heated to a temperature of from about1000° C. to about 1400° C.

The dried component may then be sintered to produce a componentcomprising an environmental barrier coating. Sintering can serve tosimultaneously densify and impart strength to the coating. Additionally,in the case of the transition and outer layers of the EBC, sintering canimpart a hermetic seal against high temperature steam present in theengine environment. Sintering can be carried out using a conventionalfurnace, or by using such methods as microwave sintering, lasersintering, infrared sintering, and the like.

Sintering can be accomplished by heating the dried component at a rateof from about 1° C./min to about 15° C./min to a temperature of fromabout 1100° C. to about 1700° C. and holding the component at thattemperature for from about 0 to about 24 hours. In another embodiment,sintering can be accomplished by heating the coated component at a rateof from about 5° C./min to about 15° C./min to a temperature of fromabout 1300° C. to about 1375° C. and holding the component at thattemperature for from about 0 to about 24 hours. In another embodiment,sintering can occur rapidly by placing the coated component into afurnace heated to a temperature of from about 1000° C. to about 1400° C.

Binder burnout and sintering heat treatments may be carried out in anambient air atmosphere, or in an inert gas atmosphere where the inertgas is selected from hydrogen, a noble gas such as helium, neon, argon,krypton, xenon, or mixtures thereof. In one embodiment, the inert gasatmosphere can be used in conjunction with Lnb and SiO₂ sintering aidsso as not to convert the rare earth metal to an oxide before it melts.Maintaining the Lnb metal in a metal state can promote liquid phasesintering and subsequent reaction with the SiO₂.

In an alternate embodiment, all layers of the EBC can be applied, one ontop of the other, before masking removal, organic processing aidburnout, and sintering are carried out. Those skilled in the art willunderstand that after application of each layer, the layer should bedried, or partially dried, before the application of the subsequentlayer.

In another embodiment, the sintering aid does not need to be addeddirectly to the transition or outer layer of the slurry to achieve thedesired result. The sintering aid can be added to one layer of the EBCslurry and during sintering, the sintering aid can diffuse throughoutthe EBC slurry to the remaining layers. In another embodiment, a primarymaterial slurry with no sintering aid can be densified by applying thelayer, allowing it to dry, and then back infiltrating a sol-gel solutioncomprising a sintering aid prior to heat treatment as explained below.

Infiltration may allow for the densification of a thicker layer of EBCmaterial at one time. Moreover, infiltration is a way to add moresintering aid after sintering if the coating isn't as dense as desired.The sol-gel solution used for infiltration may be an aqueous solution ofa water soluble salt sintering aid, as defined previously, or a solutionof an organic solvent and a solvent soluble salt sintering aid.

As used herein, “organic solvent” refers to methanol, ethanol, propanol,butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol,dodecanol, acetone, methyl isobutyl ketone (MIBK), methyl ethyl ketone(MEK), toluene, ethylbenzene, propyl benzene, methoxybenzene, heptane,octane, nonane, decane, xylene, mineral spirits, naptha (such as VM&Pnaptha), tetrahydrofuran, ethers, and combinations thereof.

As used herein, “solvent soluble salt sintering aids” can includesolvent soluble iron salts, solvent soluble gallium salts, solventsoluble aluminum salts, solvent soluble nickel salts, solvent solubletitanium salts, solvent soluble boron salts, and solvent solublealkaline earth salts. More specifically, as used herein,“solvent-soluble iron salts” can include ethoxide, iron2,4-pentanedionate, and iron tetramethylheptanedionate; “solvent-solublegallium salts” can include gallium 8-hydroxyquinolinate, gallium2,4-pentanedionate, gallium ethoxide, gallium isopropoxide, and gallium2,2,6,6-tetramethylheptanedionate; “solvent-soluble aluminum salts” caninclude butoxide, aluminum di-s-butoxide ethylacetoacetate, aluminumdiisopropoxide ethylacetoacetate, aluminum ethoxide, aluminumethoxyethoxyethoxide, aluminum 3,5-heptanedionate, aluminumisopropoxide, aluminum 9-octadecenylacetoacetate diisopropoxide,aluminum 2,4-pentanedionate, aluminum pentanedionatebis(ethylacetoacetate), aluminum 2,2,6,6-tetramethyl-3,5-heptanedionate,and aluminum phenoxide; “solvent-soluble nickel salts” can includenickel 2,4-pentanedionate, nickel2,2,6,6-tetramethyl-3-5-heptanedionate; “solvent-soluble titanium salts”can include titanium allylacetoacetatetriisopropoxide, titaniumbis(triethanolamine)diisopropoxide, titanium butoxide, titaniumdi-n-butoxide bis(2-ethylhexanoate), titaniumdiisopropoxide(bis-2,4-pentanedionate), titanium diisopropoxidebis(tetramethylheptanedionate, titanium ethoxide, titaniumdiisopropoxide bis(ethylacetoacetate), titanium 2-ethylhexoxide,titanium iodide triisopropoxide, titanium isobutoxide, titaniumisopropoxide, titanium methacrylate triisopropoxide, titaniummethacryloxyethylacetoacetate triisopropoxide, titanium methoxide,titanium methoxypropoxide, titanium methylphenoxide, titaniumn-nonyloxide, titanium oxide bis(pentanedionate), titanium oxidebis(tetramethylheptanedionate), and titanium n-propoxide;“solvent-soluble boron salts” can include boron ethoxide, boronbutoxide, boron isopropoxide, boron methoxide, boron methoxyethoxide,boron n-propoxide; and “solvent-soluble alkaline earth salts” caninclude calcium isopropoxide, calcium methoxyethoxide, calciummethoxide, calcium ethoxide, strontium isopropoxide, strontiummethoxypropoxide, strontium 2,4-pentanedionate, strontium2,2,6,6-tetramethyl-3,5-heptanedionate, magnesium ethoxide, magnesiummethoxide, magnesium methoxyethoxide, magnesium 2,4-pentanedionate,magnesium n-propoxide, barium isopropoxide, barium methoxypropoxide,barium 2,4-pentanedionate, barium2,2,6,6-tetramethyl-3,5-heptanedionate.

As used herein, “sintering aid(s)” refers to any of a “slurry sinteringaid,” a “water soluble sintering aid,” or a “solvent soluble saltsintering aid,” as defined previously. Without intending to be limitedby theory, the inclusion of sintering aids to the EBC embodiments hereincan increase the rate of diffusion of the primary material such thatsurface area reduction (i.e. high surface area particles consolidatingto form a dense coating) can occur at lower temperatures than it wouldhave absent the sintering aid. As previously described, sintering atlower temperatures (i.e. about 1357° C. or below) can not only result ina highly dense (i.e. greater than about 85% for the transition layer,greater than about 70% for the compliant layer, and greater than about70% for the outer layer) coating that can be less susceptible to thepenetration of hot steam from the engine environment, but can also helpprevent the degradation of the mechanical properties of the underlyingcomponent that could result from prolonged exposure to highertemperatures.

Sintering aids can act in a variety of ways depending on the amount ofsintering aid included in the EBC and the time at which the coating isexposed to sintering temperatures. For example, in one embodiment, thesintering aid can dissolve completely into the primary material (i.e.primary transition, outer, or compliant, materials) to “dope” thematerial. In another embodiment, if the amount of sintering aid that issoluble in the primary material is exceeded, the remaining insolubleportion of sintering aid can react with the primary material to form thesecondary material (i.e. secondary transition, compliant, or outermaterial). In another embodiment, primary material and secondarymaterial can be present as described previously, along with residualsintering aid.

In these latter two embodiments, when the secondary material is highlyvolatile in high temperature steam, such as but not limited to, alkalisilicates, alkaline earth silicates, mullite, iron silicate,borosilicate glass, nickel silicate, and residual sintering aids ofiron, aluminum, titanium, gallium, nickel, boron, alkali, andalkali-earth compounds, as long as the total volume of secondarymaterial, plus porosity (plus residual sintering aid when present) ineither of the intermediate layer or compliant layer (when present) ofthe EBC remains about 15% by volume or less, the hermetic seal can bemaintained. Alternately, in these latter two embodiments, when thesecondary material is highly resistant to volatilization in hightemperature steam, such as when the secondary material comprises a rareearth containing compound, such as but not limited to rare earth oxide,rare earth titanate, rare earth iron compound, rare earth gallate, rareearth aluminate, and rare earth aluminosilicate, the porosity in eitherof the intermediate or compliant layer (when present) of the EBC needremain about 15% by volume or less to maintain the hermetic seal.

It should be noted that at low levels of sintering aid, the densifiedcoating layer might not initially include any detectable secondarymaterials. In some embodiments, the secondary materials may never becomedetectable. In other embodiments, however, after hours of exposure tohigh temperature steam in the engine environment, the secondarymaterials can become detectable using techniques such as x-raydiffraction, electron microscopy, electron dispersive spectroscopy, andthe like.

EBC embodiments described herein can offer a variety of benefits overcurrent EBCs and manufacturing processes thereof. Specifically, aspreviously described, the inclusion of a sintering aid in the EBCembodiments herein can permit sintering at lower temperatures (i.e.about 1357° C. or below). This can result in a highly dense (i.e.greater than about 85% for the transition layer, and greater than about70% for each of the outer, and compliant, layers) coating that can beless susceptible to the penetration of hot steam from the engineenvironment, and can also help prevent the degradation of the mechanicalproperties of the underlying component that could result from prolongedexposure to higher temperatures. Also, the embodiments set forth hereincan be made at less expense than current EBCs due to the use of theslurry deposition process, which is made possible by the incorporationof sintering aids into the various layers. Moreover, the presentembodiments can provide for EBCs having a more uniform thickness thanconventional techniques, such as plasma spraying, even when applyingthin layers (<2 mils or less than about 50 micrometers). Additionally,the slurry deposition process can allow for the application of the EBCsto internal component passages as well as the ability to produce smoothsurface finishes without an additional polishing step.

There can be occasions when the EBC develops small and/or narrow defects(e.g. about 10 microns to about 5 mm in diameter; or about 10 microns toabout 1 mm in width) that need to be repaired. The following repairprocesses are applicable to the EBCs described herein and may be carriedout after sintering of an individual EBC layer, or after sintering theentire applied EBC, as explained herein below.

In one embodiment, repairs may include remedying defects in one or moreindividual layers as the EBC is being applied using the methodsdescribed herein. In this embodiment, the repair can be carried outafter sintering a given layer by applying a repair slurry comprising thesame slurry materials used to make the layer having the defects. Forexample, if the transition layer develops a defect after sintering, thedefect could be repaired using a “transition layer repair slurry” thatcomprises the same transition layer slurry materials used in theoriginal application of the transition layer. In one embodiment, therepair slurry can comprise a higher solids loading of primary materialceramic particles than the original slurry layer as this can reduceshrinkage on drying and sintering of the repaired portion of thecoating. In particular, the solids loading of primary material ceramicparticles in the repair slurry can be greater than about 30% to about55% by volume (as opposed to greater than about 10% by volume in oneembodiment of the original slurry, and from about 10% to about 55% byvolume in another embodiment of the original slurry used to make thelayer). The repair slurry may be applied using any conventional methodincluding those described previously, and the resulting “repair(ed)coating” may then be processed as described previously herein beforeapplication of any subsequent layer of the EBC.

In an alternate embodiment, repairs may include fixing defects afterapplication and sintering of the entire EBC. In this embodiment, therepair may be carried out on the EBC having defects using a transitionlayer repair slurry comprising the same materials present in thepreviously defined transition layer slurry (i.e. primary transitionmaterial, a sintering aid, and optionally secondary material). Thisparticular repair slurry can seep into any defects present in the EBCand provide a hermetic seal to the repaired EBC coating after sintering.Again, the solids loading of the transition layer repair slurry maycomprise upwards of about 30% to 55% by volume.

Additionally, repair processes may be used to reduce surface roughnessof a plasma sprayed EBC having any composition. Specifically, if thesurface roughness of a plasma sprayed EBC is unacceptable the coatingcan be smoothed over by applying either of the previously describedtransition layer slurry or outer layer slurry. When applied over theplasma sprayed EBC, the transition layer slurry or outer layer slurrycan fill in any gaps, grooves, or uneven portions of the plasma sprayedcoating and reduce the surface roughness to an acceptable degree. Morespecifically, depending on the thickness of the transition layer slurryor outer layer slurry, surface roughness of the plasma sprayed EBC canbe reduced from greater than 200 micro inch (about 5 micrometers) Ra, tobetween 40 micro inch Ra and 200 micro inch (about 1 to about 5micrometers) Ra in one embodiment, and from between 40 micro inch Ra to150 micro inch (about 1 to about 3.8 micrometers Ra in anotherembodiment. In one embodiment, the transition layer slurry or outerlayer slurry can comprise a thickness of at least about 0.5 mils (about12.5 micrometers), and in another embodiment from about 0.5 mils toabout 3 mils (about 12.5 to about 75 micrometers). The appliedtransition layer slurry or outer layer slurry can then be processed asdescribed previously to produce a repaired EBC having an acceptablesurface roughness. Additional slurry layers may be applied to the EBC ifdesired.

Such repair processes can provide the ability to repair localizeddefects, at varying points during the application or life of thecoating, as opposed to stripping off and reapplying the entire coating.This, in turn, can result in a savings of time, labor, and materials.

EXAMPLE Example 1

A silicon bond coat was applied to a SiC—SiC CMC using a conventionalair plasma spray process. Next, a primary transition material slurry wasmade by first mixing yttrium disilicate powder, aluminum oxide powder,water, polyacrylic acid-polyethylene oxide copolymer, Surfynol 502®, andglycerin in a plastic container, along with enough 0.25 inch (6.35 mm)diameter, cylindrical alumina media to line the bottom of container.This mixture was placed on a roller mill for 15 hours. After taking thecontainer off of the roller mill, the alumina media was removed. Xanthangum was then added and the mixture was shaken for 15 minutes using apaint shaker. Finally, Rhoplex® HA8 emulsion was added and the containerwas placed back onto the roller mill for 1 hour (without media).

The resulting primary transition material slurry (Slurry A) consisted of65.87% yttrium disilicate (primary transition material), 4.85% aluminumoxide (sintering aid), 6.59% polyacrylic acid-polyethylene oxidecopolymer (dispersant), 0.08% Surfynol 502® (surfactant), 0.13% xanthangum (thickener), 4.08% Rhoplex® HA8 emulsion (latex), 2.78% glycerin(plasticizer), and the balance water (all percents by weight). Thesilicon-coated ceramic component was dipped into Slurry A, dried inambient conditions, and heat-treated at 3° C./minute to 1000° C. to burnout the binder. Then, the component was sintered by heating thecomponent at 5° C./minute from 1000° C. to 1344° C. and holding for 5hours to form the transition layer.

Next, a primary compliant material slurry was made by first mixing BSASpowder, yttrium oxide powder, water, polyacrylic acid-polyethylene oxidecopolymer, Surfynol 502®, and glycerin in a plastic container, alongwith enough 0.25 inch (6.35 mm) diameter, cylindrical alumina media toline the bottom of container. This mixture was placed on a roller millfor 15 hours. After taking the container off of the roller mill, thealumina media was removed. Xanthan gum was then added and the mixturewas shaken for 15 minutes using a paint shaker. Finally, Rhoplex® HA8emulsion was added and the container was placed back onto the rollermill for 1 hour (without media).

The resulting primary compliant material slurry (Slurry B) consisted of44.62% BSAS (primary compliant layer material), 17.20% yttrium oxide(sintering aid), 6.18% polyacrylic acid-polyethylene oxide copolymer(dispersant), 0.10% Surfynol 502® (surfactant), 0.17% xanthan gum(thickener), 7.15% Rhoplex® HA8 emulsion (latex), 5.13% glycerin(plasticizer), and the balance water (all percents by weight). Thesilicon- and transition-layer coated ceramic component was dipped intoSlurry B, dried in ambient conditions, and heat-treated at 3° C./minuteto 1000° C. to burn out the binder. Then, the component was sintered byheating the component at 5° C./minute from 1000° C. to 1344° C. andholding for 5 hours to form the compliant layer.

FIG. 2. shows a SEM micrograph of a CMC (101) having this coatingmicrostructure with the air plasma spray silicon bond coat (100),transition layer (102), and compliant layer (104). The transition layer(102) is comprised of aluminum-doped yttrium disilicate primary material(106) (bright phase, see higher magnification SEM micrograph in FIG. 3),a mullite secondary material (108) (gray phase), and porosity (110)(black regions). The mullite secondary material is volatile in steam;thus, because the sum of the porosity and mullite content just exceeds15% by volume, the transition layer is not likely a hermetic barrier tohigh temperature steam. The compliant layer, in this example, impartshermeticity to the system. This layer contains yttrium-doped BSASprimary material (gray phase), yttrium disilicate steam secondarymaterial (bright phase), mullite secondary material (dark gray phase),and porosity (black region). Here, only the mullite secondary materialhas high volatility in steam, and the combined amount of mullite andporosity is less than 15% by volume.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

What is claimed is:
 1. A component comprising: a plasma sprayedenvironmental barrier coating having a surface characterized by asurface roughness; and a sintered layer overlying the plasma sprayedenvironmental barrier coating and defining an outer surface having alower surface roughness than the surface of the environmental barriercoating, the sintered layer being formed from a slurry applied to andthen sintered on the plasma sprayed environmental barrier coating, thesintered layer comprising a primary material, at least one sintering aiddissolved in the primary material, and optionally a secondary material,wherein: the at least one sintering aid contains at least one dopingcomposition chosen from the group consisting of iron, aluminum,titanium, gallium, nickel, boron, alkali metals, alkaline-earth metals,and Lnb rare earth metals, and the at least one sintering aid isselected from the group consisting of iron oxide, gallium oxide,aluminum oxide, nickel oxide, titanium oxide, boron oxide, alkalineearth oxides, carbonyl iron, iron metal, aluminum metal, boron, nickelmetal, iron hydroxide, gallium hydroxide, aluminum hydroxide, nickelhydroxide, titanium hydroxide, alkaline earth hydroxides, ironcarbonate, gallium carbonate, aluminum carbonate, nickel carbonate,boron carbonate, alkaline earth carbonates, iron oxalate, galliumoxalate, aluminum oxalate, nickel oxalate, titanium oxalate, watersoluble iron salts, water soluble gallium salts, water soluble aluminumsalts, water soluble nickel salts, water titanium salts, water solubleboron salts, water soluble alkaline earth salts, and a combinationcomprising an Lnb rare earth metal and SiO₂; the primary material is arare earth disilicate or a rare earth monosilicate, and the primarymaterial is doped with the at least one doping composition of the atleast one sintering aid so as to be either a doped rare earth disilicatecontaining the at least one doping composition or a doped rare earthmonosilicate containing the at least one doping composition; and thesecondary material optionally in the sintered layer is a reactionproduct of the primary material and any of the at least one sinteringaid not dissolved in the primary material.
 2. The component of claim 1,wherein the at least one sintering aid consists of one or more of theiron oxide, gallium oxide, aluminum oxide, nickel oxide, titanium oxide,boron oxide, alkaline earth oxides, carbonyl iron, iron metal, aluminummetal, boron, nickel metal, iron hydroxide, gallium hydroxide, aluminumhydroxide, nickel hydroxide, titanium hydroxide, alkaline earthhydroxides, iron carbonate, gallium carbonate, aluminum carbonate,nickel carbonate, boron carbonate, alkaline earth carbonates, ironoxalate, gallium oxalate, aluminum oxalate, nickel oxalate, titaniumoxalate, water soluble iron salts, water soluble gallium salts, watersoluble aluminum salts, water soluble nickel salts, water titaniumsalts, water soluble boron salts, and water soluble alkaline earthsalts.
 3. The component of claim 1, wherein the at least one sinteringaid consists of the Lnb rare earth metal and SiO₂.
 4. The component ofclaim 3, wherein the Lnb rare earth metal is selected from the groupconsisting of lanthanum, cerium, praseodymium, neodymium, promethium,samarium, europium, gadolinium, terbium, dysprosium, and ytterbium. 5.The component of claim 1, wherein the sintered layer is an outer layeron the plasma sprayed environmental barrier coating and the primarymaterial is the doped rare earth monosilicate.
 6. The component of claim5, wherein the outer layer contains the secondary material, and thesecondary material is at least one selected from the group consisting ofFe₂O₃, iron silicates, rare earth iron oxides, Al₂O₃, mullite, rareearth aluminates, rare earth aluminosilicates, TiO₂, rare earthtitanates, Ga₂O₃, rare earth gallates, NiO, nickel silicates, rare earthnickel oxides, Lnb metals, Lnb₂O₃, Lnb₂Si₂O₇, Lnb₂SiO₅, borosilicateglass, alkaline earth silicates, alkaline earth rare earth oxides,alkaline earth rare earth silicates, and mixtures thereof.
 7. Thecomponent of claim 5, wherein the outer layer consists of about 85% toabout 100% by volume of the rare earth monosilicate doped with the atleast one doping composition and in which the at least one sintering aidis dissolved, and any balance is the secondary material.
 8. Thecomponent of claim 5, wherein the outer layer has a porosity of about 0%to about 30% by volume.
 9. The component of claim 5, wherein the outersurface of the outer layer is the outermost surface of the component.10. The component of claim 1, wherein the sintered layer is a transitionlayer on the plasma sprayed environmental barrier coating and theprimary material is the doped rare earth disilicate.
 11. The componentof claim 10, wherein the transition layer contains the secondarymaterial, and the secondary material is at least one selected from thegroup consisting of Fe₂O₃, iron silicates, rare earth iron oxides,Al₂O₃, mullite, rare earth aluminates, rare earth aluminosilicates,TiO₂, rare earth titanates, Ga₂O₃, rare earth gallates, NiO, nickelsilicates, rare earth nickel oxides, Lnb metals, Lnb₂O₃, Lnb₂Si₂O₇,Lnb₂SiO₅, borosilicate glass, alkaline earth silicates, alkaline earthrare earth oxides, alkaline earth rare earth silicates, and mixturesthereof.
 12. The component of claim 10, wherein the transition layerconsists of about 85% to about 100% by volume of the rare earthdisilicate doped with the at least one doping composition and in whichthe at least one sintering aid is dissolved, and any balance is thesecondary material.
 13. The component of claim 10, wherein thetransition layer has a porosity of from 0% to about 15% by volume. 14.The component of claim 10, further comprising a second sintered layer onthe plasma sprayed environmental barrier coating, the second sinteredlayer being an outer layer overlying the transition layer and definingan outermost surface of the component, the outer layer containing a rareearth monosilicate, the at least one sintering aid dissolved in the rareearth monosilicate, and optionally the secondary material, the rareearth monosilicate being doped with the at least one doping compositionof the at least one sintering aid so as to be a doped rare earthmonosilicate containing the at least one doping composition.
 15. Thecomponent of claim 14, further comprising a third sintered layer on theplasma sprayed environmental barrier coating, the third sintered layerbeing a compliant layer between the transition layer and the outerlayer, the compliant layer containing BSAS, the at least one sinteringaid dissolved in the BSAS, and optionally a secondary compliant materialselected from the group consisting of Ln₂O₃, Ln₂Si₂O₇, Ln₂SiO₅,Ln₃Al₅O₁₂, Al₂O₃, mullite, and combinations thereof, the BSAS beingdoped with the at least one doping composition of the at least onesintering aid so as to be a doped BSAS containing the at least onedoping composition.
 16. The component of claim 15, wherein the compliantlayer contains the secondary compliant material.
 17. The component ofclaim 15, wherein the compliant layer consists of about 85% to about 99%by volume of the BSAS doped with the at least one doping composition andin which the at least one sintering aid is dissolved, and the balance isabout 1% to about 15% by volume of the secondary compliant material. 18.The component of claim 15, wherein the compliant layer has a porosity ofabout 0% to about 30% by volume.
 19. The component of claim 1, whereinthe surface roughness of the environmental barrier coating is greaterthan 5 micrometers Ra and the lower surface roughness of the outersurface of the sintered layer is 1 to 5 micrometers Ra.
 20. Thecomponent of claim 1, wherein the component is a ceramic matrixcomposite or a monolithic ceramic turbine engine component selected fromthe group consisting of combustor components, turbine blades, shrouds,nozzles, heat shields, and vanes.